Dogs come in all shapes and sizes. In size they range from "toy" to "giant," and the sizes of individual dogs form a continuum from smallest to largest. Some differences between "medium" and "large" dog breeds, such as weight and height, are obvious. Other differences aren't as obvious, such as how the purpose of the breed guided selection for size; health issues linked to size; or the genetics that actually determine size.
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Weight
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One of the biggest obvious differences between medium and large dog breeds is how much the dogs weigh. Medium dog breeds typically weigh between 25 and 50 pounds. Dogs that are considered large breeds usually weigh between 50 and 75 pounds.There are exceptions for these weight averages, however. For example, the Cocker Spaniel is considered a medium dog but can weigh as little as 15 pounds. The Bull Terrier is also considered a medium dog but can weigh up to 80 pounds. These dogs are still considered medium dog breeds because their average weight falls within the range for medium dog breeds.
Height
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Dog height is also part of how we differentiate between medium and large dog breeds. Height of dogs is measured from the paw to the top of the shoulder. Large dog breeds are typically between 25 and 27 inches. Medium dogs usually stand about 20 inches. Just like with weight, there are some dog breeds that are smaller or larger than the average height, yet are still considered either a medium or large breed dog. But this is just how humans have classified "medium" and "large" dogs. The National Human Genome Research Institute found in a study of differences in dog size that variations of a single gene create differences in dog stature. This gene is called IGF-1, or insulin-like growth factor-1, which is a protein hormone.
Health Issues
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Diseases and other health problems become more common as dogs get older. Some health issues, however, are more common as dogs get larger. These problems include bone and joint issues. Bone and joint problems can be caused by poor-fitting joint sockets. The size of the bones, as well as how much more body weight a large-breed dog has to carry around compared with smaller breeds, puts more strain on joints and bones. Many bone and joint issues can be eased with a proper diet. Another common health problem for large-breed dogs is bloat. This problem is caused by the twisting of the stomach, causing the passages to and from the stomach to be pinched off. The stomach fills with gas, which in short order makes it press against the heart and other organs. This is rapidly fatal if not discovered and urgently treated. Even healthy large dogs tend to have shorter lives than healthy medium and small dogs.
Purpose of the Breed
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Dogs have been used as working companions throughout their association with humans. They have been selected for the types of work humans needed to do, and size has been one strong factor in selection for various jobs. Medium dog breeds are known for their agility and speed. Breeds such as collies, sheep dogs and Australian shepherds were developed from dogs used for herding livestock. Other medium breeds, such as spaniels and hounds, are from stock originally bred as hunting dogs. Very large dogs like the Bernese Mountain Dog can carry or pull considerable weight. Dogs often have been used to pull weight, such as carts or sleds, over the course of their association with humans. Some large breeds such as German shepherds were developed from ancestors selected for qualities that would serve well in protecting herds from predators. Dogs like the Akita or Doberman Pinscher have been bred as guard dogs and are quite effective due to their size and protective nature.
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